In a mass spectrometer, a sample as an object is irradiated with a laser beam to ionize the sample in a vacuum chamber, the ionized substance is extracted by an extraction electrode to which a high voltage is applied, and accelerated. Based on the mass-to-charge ratio of the ions calculated from the flight time of the ions, a mass spectrum is obtained. In the mass spectrometer, an imaging process may be performed in which mass spectral information on each measurement point on the sample is acquired while the sample is moved in a horizontal plane in the vacuum chamber, and an image of a molecular distribution in the sample is displayed based on the acquired mass spectrum information. In this case, when the height of the measurement point varies from point to point, the accuracy of process is impaired. This is because, when the height of the sample in the vacuum chamber is deviated from a predetermined position, the position from which the ion starts to fly is deviated from the predetermined position, and the flight time is not correctly measured (and therefore the mass-to-charge ratio is not correctly identified).
When an object is to be moved in a horizontal plane in a vacuum chamber, like the mass spectrometer that performs the imaging, in many cases, a guide rail is laid on a floor of the vacuum chamber, a stage that holds the object is disposed on the guide rail, and a driving mechanism that drives the stage is coupled to the stage. According to this configuration, the stage that holds the object is moved while guided by the rails laid on the floor, so that the positional deviation in the height direction of the object hardly occurs.
In general, in order to maintain airtightness of a vacuum chamber, the mechanism that moves the stage in the vacuum chamber and the driving source such as a motor are accommodated within the vacuum chamber. However, a component that generates heat, such as a motor, easily rises in temperature in a vacuum chamber because no heat discharging means exists in the vacuum chamber except for heat conduction. For example, Patent Literatures 1 and 2 disclose a configuration in which a motor that drives the stage is disposed outside the vacuum chamber.